Bowser's right--the current technology, often used for pets and livestock, basically squawks a number at you if you pass a wand or something over it. It has no inherent transmission ability. I don't know how large an interrogation device could be made, or what it's maximum range could be. I could easily see something like an airport detector gate that could read such chips.

Still, given the size of cell phones and miniaturized GPS receivers, I could also see systems that could transmit to cell phone towers, and provide an actual location based on GPS data. I would venture to guess that something like this could be made now that would be about the size of a large pacemaker, and have similar power requirements. Awfully big for a routine implant. As for miniature ones, not yet, but who knows?